Generally, a ballast tank for storing ballast water is installed in ships, for adjusting the balance and sea gauge of each of the ships.
Freight is loaded on a ship, and when the front-back balance and/or left-right balance of the ship are/is not made, the ship may be balanced by supplying the ballast water to the ballast tank or discharging the ballast water from the ballast tank. Also, ships are generally designed to economically and stably sail when freight is loaded thereon. Therefore, a condition similar to a freight-loaded state is provided to a ship by adjusting the amount of ballast water stored in the ballast tank according to the amount of loaded freight, thereby enabling the ship to stably sail.
Here, the ballast water stored in the ballast tank is adjusted by supplying or discharging seawater/fresh water, located near the ship, to or from the ballast tank with a pump. However, foreign substances (i.e., inorganic substances such as pebbles, sand, and mud, creatures such as fishes and shells, shellfish, and plankton, and organic substances) in a ship-located area can flow into the ballast tank in a process of supplying the ballast water to the ballast tank. Therefore, when the ship sails at a long distance, the foreign substances are precipitated and stuck, causing a reduction in a function of the ballast tank. Also, when the ballast water is discharged from the ballast tank while freight is being loaded on the ship, creatures and organic substances included in the ballast water are discharged together, and for this reason, the surrounding ecosystem is changed, causing an adverse effect on an ambient environment.
Therefore, international maritime organization (IMO) has introduced the ballast water purification rule in 2004, for preventing an ecosystem from being damaged due to ballast water. Accordingly, ships which sail at a long distance should be essentially equipped with a device for removing foreign substances such as creatures or organic substances included in ballast water. Also, a large amount of ballast water is used in large ships such as oil tankers, containers, and LNG carriers which sail at a long distance.
Therefore, it is required to develop an efficient filter device for quickly filtering out a large amount of seawater or fresh water which is used as ballast water for large ships. Also, it is required to develop a filter device that quickly backwashes a filter member while filtering out foreign substances such as microbes included in seawater or fresh water, thereby enhancing the filtering efficiency of the filter member.